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By Anne Wainscott-Sargent, AIAA Communications Team

As the aviation sector looks to achieve net zero carbon emissions by 2050, the biggest gains may not happen in the air but on the ground, stated Tim Lieuwen, the 2025 AIAA Dryden Lecturer in Research, during the 2025 AIAA SciTech Forum in January.

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“The least cost way to get to a net-zero society is to take a system view about economy-wide CO2 emissions and where and how aviation fits into that, rather than trying to zero out CO2 emissions sector by sector.  It makes sense if you think about it – it’s a whole lot cheaper to manage your CO2 emissions from something that’s sitting on the ground, potentially sitting right above a depleted oil reservoir versus trying to manage something that’s flying around and has to deal with all the safety issues of aviation,” said Lieuwen.

The Georgia Tech executive vice president for Research, Regents’ Professor, holder of the David S. Lewis, Jr. Chair, and the executive director of the Strategic Energy Institute  explored the interconnectedness of energy sources, carriers, and storage systems, noting the significant role of fossil fuels in the current U.S. energy system and the potential for synthetic fuels.

He highlighted four different options for zeroing out climate impacts using a high-fidelity model of the entire energy system. Organized in a 2×2 matrix, the model showed the option of economy-wide decarbonization, where different sectors contribute in a coordinated way. Then he presented a visual of sector-by-sector decarbonization, where each sector individually zeroes out its contributions.

According to the EPA, transportation is the largest contributor of CO2 emissions, with the aviation sector contributing roughly 2.5%, compared with 28% from automobiles.

Lieuwen noted there is a difference between zero CO2 and net zero. Net zero focuses on the overall CO2 emissions budget, allowing for some sectors to potentially emit CO2 and some sectors to be net-negative CO2.  In this scenario, the least-cost role of aviation in an economy wide net-zero CO2 society is a mix of conventional fossil fuels and renewable hydrocarbons like sustainable aviation fuels (SAF). If aviation’s aim is to pursue “a least-cost societal net-zero target,” then he advocated for an economy-wide net zero strategy.

Using a least-cost model, the energy expert showed some surprising insights where fossil fuels and renewable fuels are equally split 50/50.

“Half are fossil fuels and the other half are synthetic fuels that you can manufacture like SAF. You see big growth in renewables and big growth in biofuels,” he explained.

Lieuwen also observed that in this least-cost world, half of all energy will rely on electricity which will prompt big growth in electrification, going from 20% to 50%. He also predicted significant R&D investments around power electronics, high-voltage motors, batteries, and energy storage.

Fossil Fuels Dominate Current Energy Economy
Another big takeaway was how society moves and stores energy will continue to use  fossil fuels, although in a significantly diminished role from today.

“We’re in an 80/20 split with the current U.S. energy economy as a whole, which means that we use fossil fuels roughly for 80% of the means by which we move energy around and store it. We use electricity as an energy carrier for the other 20%. These are multi-trillion-dollar sectors. It’s important to recognize the interconnectedness of all this. For example, the aviation sector is leveraging and contributing technologically to and is also benefiting from infrastructure of existing industrial sectors, such as oil pipelines and the oil refining industry.”

Aviation’s Critical Role
Part of achieving this least-cost societal net-zero target in aviation is developing SAF, which currently are more expensive than fossil fuel, and will likely require policy levers, carbon taxes, or tax credits to become a reality, Lieuwen predicted.

There will continue to be a premium placed on aviation advances that offer thermal efficiency as well as operational flexibility.

“The ability to have systems that are low emission/high efficiency, but yet don’t surge/don’t stall, where your flame stays attached, where the system is stable, is very, very important,” said the researcher before briefly sharing highlights of his research that focuses on better understanding the interaction of how fast waves of flames move in combustion engines.

“The interaction of acoustic waves… create interference patterns which are controlled by how fast vortices move versus how fast waves on flames move,” he explained. “If a vortex is not moving at the same speed, what’s happening is you have two periodical disturbances moving at different velocities.”

This phenomenon leads to destructive instabilities in rockets, in home heaters, and in aircraft engines, Lieuwen shared.

Asked after his presentation if he thought the increased tempo in rocket launches would hurt efforts to decarbonize, Lieuwen said, “I would suspect the overall carbon footprint that is going to those direct launches will pale relative to other sectors.” He predicted major follow-on secondary impacts from all the satellite activity, however.

Nuclear’s Potential
Another question concerned the role of nuclear energy in getting to net zero. “Nuclear is really important,” said Lieuwen. “In fact, if we could solve this problem of low-cost nuclear [energy] it would totally transform what least-cost net zero looks like.”

Amanda Simpson, former deputy assistant Secretary for Energy under the Obama administration who also directed the U.S. Army Office of Energy Initiatives, found Lieuwen’s remarks timely and on target. The former VP for Research and Technology and head of Sustainability for Airbus Americas said that the aviation sector has grappled with the question of whether net zero by 2050 is the right commitment.

“While it’s an admirable goal, is it a realistic? It’s a very expensive and difficult goal,” she said.
Simpson added that addressing the CO2 issue in aviation is also hard, and she agreed with Lieuwen that it’s easier to decarbonize something on the ground.

“There’s so much to be done in the remaining 26 years, we have to go after everything. There is not going to be a silver bullet – we have to tackle everything to start bringing the [greenhouse gas usage] totals back,” she said.

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